A ProSe feature specified by 3GPP Technical Specification (TS) 23.303, July 2015 allows for ProSe Direct Discovery and ProSe Direct Communication, which enable user equipments (UEs) to discover and communicate with each other directly rather than routing data via an evolved Node B (eNodeB). This can offer high data rates and low end-to-end delays as a result of the short range direct communication. D2D also allows for range-extension via UE-to-UE relaying. In this specification D2D and ProSe can be used interchangeably. A direct radio link between two or more UEs is known as a “sidelink” (see 3GPP TS 36.211), to distinguish it from conventional uplink (UL) and downlink (DL) connections between UE and eNodeB. Sidelink (SL) communications use a subset of the LTE/LTE-A UL time-frequency resources and use Single Carrier-Frequency Division Multiple Access (SC-FDMA), i.e., the same transmission scheme as LTE/LTE-A uplink transmissions. The relevant SL channels comprise: Physical Sidelink Control Channel (PSCCH) for SL control information; Physical Sidelink Shared Channel (PSSCH) for SL data; Physical Sidelink Discovery Channel (PSDCH) for discovery announcements and Physical Sidelink Broadcast Channel (PSBCH) for broadcast of D2D system information to assist D2D synchronization.
It is desirable to be able to efficiently allocate available wireless network resources and to appropriately manage contention for network resources. It is known to use Quality-of-service Class Identifiers (QCIs) in LTE/LTE-A UL and DL communications to indicate packet priority and these QCIs are assigned between a UE and an Evolved Packet Core (EPC) via System Architecture Evolution (SAE) bearers at a Packet Data Convergence Protocol (PDCP) protocol layer. There is an aim to enable priority support for D2D or ProSe communication to prioritize access to available radio resources according to, for example, traffic characteristics. One application of D2D communication is in implementing LTE-based public safety networks, in which it will be appreciated that there is likely to be an aim for more important network traffic to be prioritized over less important network traffic. The more important traffic can be, for example, a voice of a firefighters' commander which can have a higher priority than voice data of the lower-ranking firefighters. Compared to commercial networks, public safety networks can have more stringent target service criteria for reliability and security and are likely to implement D2D communication, for example, when wireless cellular coverage fails or is not available. However, prioritization of network resources is also desirable in commercial networks to allow prioritization of traffic for different services like voice, video and data. Thus there is a desire to provide priority handling support for SL communication.